A secondary battery has been used as a source of power supply for portable devices such as mobile phones, digital cameras, PDAs, and notebooks. With the increasing global awareness of the problems associated with the depletion of petroleum resources and global warming, the demand for middle or large size-secondary batteries applicable to hybrid electric vehicles (HEVs), electric tools, electric motorcycles, robot industries, and others has sharply increased. In order to satisfy such demand, there is a need for developing an environment-friendly battery having a high output (high C-rate characteristics), high energy density, and excellent stability during repeated charging and discharging cycles.
In general, a secondary battery is composed of a cathode, an anode, an electrolyte, and a separator, and it converts a chemical energy into an electrical energy through the reaction of lithium ions reversibly intercalated between the cathode and the anode. Especially, the active materials constituting the electrodes, the positive active material and the negative active material, are the most important factors that influence the battery performance characteristics.
A carbon material has been generally used for preparing a negative active material. However, the carbon material currently commercialized (e.g., graphite) only allows, in theory, the intercalation of one lithium per 6 carbon atoms (LiC6), which gives a theoretical maximum capacity of only 372 mAh/g.
In order to overcome such capacity limitation to achieve a higher energy density, extensive studies on tin oxide, transition metal oxide-based materials, lithium, lithium alloys, carbon composite materials, and silicon-based negative active materials have been conducted. A silicon-based negative active material has a theoretical maximum capacity of 4200 mAh/g, which is 10-fold higher than that of a graphite-based negative active material. Further, tin oxide also has a high theoretical capacity of at least 700 mAh/g, but it undergoes an unacceptably large volume change (˜300%) during the charging/discharging cycles, which causes its separation from the electrode, making it difficult to maintain a sustained battery cycle performance.
In order to minimize the internal stress caused by such volume change, studies on modifying the surface of a negative active material or using fine nano-meter sized nanostructures (e.g., nanoparticles and nanowires) have been conducted. Such nanostructures include a hollow SnO2 ball structure [Advanced Materials, Vol. 18, 2325 (2006)], a SnO2 nanowire structure [Applied Physics Letters, Vol. 87, 113108 (2005)], and a SnO2—In2O3 composite nanostructure [Nano Letters, Vol 7, 3041 (2007)]. However, even in case of using such nanostructures as a negative active material, 30% discharge capacity decay has been observed after 10 cycles.
Further, there have been carried out a number of studies on a composite-typed negative active material composed of a graphite-based negative active material mixed with a silicon-based and tin oxide-based negative active materials, but as the content of the graphite-based material increases, the capacity decay becomes unacceptably high.
Example of transition metal oxides used as a positive active material are V2O5, CuV2O6, NaMnO2, NaFeO2, LiCoO2, LiNiO2, LiNi1-yCOyO2 (0≦y≦0.85), LiMn2O4, Li[Ni1/2Mn1/2]O2, LiFePO4, and transition metal oxides doped with 1 atom % or less of Mg2+, Al3+, Ti4+, Zr4+, Nb5+, or W6+ in the lithium sites of LiFePO4, and the above-mentioned positive active materials may be used in the form of a composite in order to enhance the high-output and high-capacity characteristics.
Also, a negative active material and a positive active material are applied on a collector using various methods such as screen printing, spin coating, and vacuum deposition, but there is a need for developing a specific, low-cost coating technology that does not require the use of binders or additives and can be applied to create a large area having a broad range of thickness.